Look Inside the Riflescope: A Buyer's Guide
July 10, 2014
We call it a riflescope, but the proper term is telescopic sight, which is an optical device based on a refracting telescope. An image we call the reticle is mounted inside of the tube to provide an aiming reference.
An old adage about optics says the hunter should spend as much on the scope as on the rifle. A person could make a decision based on the price tag, but just as there are advancements in optical quality, adjustment and reticles, there are compromises that impact performance. And lot of things might go wrong.
Erector springs can fail, O-rings may crack and allow moisture into the tube. Extreme recoil can shake lenses out of place. Bacteria, introduced in a factory in China, can cloud the lens in Wyoming. If you want to choose your optics like a pro, watch how the pros do it.
Cabela's Matt Highby is the optics category manager who oversees which glass gets featured in the company's retail stores, on the Internet and in the catalog. The first thing he does with a new scope is turn it around.
"I like to look through the scope from the opposite direction and look for small particles on the lenses," Highby said.
In the machining process, fine shavings of metal can be left in the tube. Sometimes that small debris can end up on the inside of a lens, creating a speck or a shadow that becomes an annoyance in the field.
Another problem, Highby said, is bacteria. If they attach to the glass, microscopic organisms can grow to cloud the optic. Unfortunately, this is one of those things that can show up later in the life of the scope. One way that some manufacturers combat this is to introduce ultraviolet light at one stage in the process, to kill the bacteria.
Another factor to be judged from the objective end is the interior finish and baffling. Manufacturers often coat the inside of the tube with a special paint that does not reflect, that allows more light through the tube.
"When you see a dull reflection, they have actually coated the surface. But any time you can see reflection, that is light lost in the scope. If you see light bouncing back, that translates to light that doesn't make it to the pupil," Highby said.
Now turn the scope around.
Through the Ocular
Tim Gardner is the owner of Alpen Optics. He likes to gauge the sharpness and clarity of the object at high magnification.
"It helps to set the scope in a fixture and see how clear the image is. I want to see how the windage and elevation adjustments work. And how the fast-focus eyepiece works."
Gardner is looking for a sharp image at high magnification and alignment of the reticle once everything is in focus. One red flag is slop in the focusing eyepiece. Another is the magnification adjustment. If the adjusting ring binds, there is a problem with the erector tube, which will get worse with recoil.
Take a look at the lens specifications. To get bright, sharp viewing with true color fidelity, manufacturers employ various phase and metallic coatings on the lenses while the air-to-glass lenses receive multiple coatings. Check for phase coatings, which promote resolution and color fidelity, and metallic prism coatings for light transmission and brightness.
Manufacturers that care about their reputation submit their scopes to torture testing. One critical area is the erector assembly, which is contained within a tube inside the scope. The erector tube encloses a collection of lenses that, adjusted by windage and elevation knobs, tilt the assembly up and down, right and left.
Because the erector tube is contained within a larger tube, the more windage adjustment that is needed to set the zero, the less vertical travel will be available and vice versa. The whole system can fail when subjected to shock or repeated heavy recoil.
"You want that point of impact to stay the same. Variable powers are most popular and there are a lot of parts. Erector tube assembly, lenses, grooves and sleeves," Gardner said.
The riflescope, no matter how well built, is still a precise optical instrument.
"With heavy recoil, you beat the crap out of the scope every time you pull the trigger. I have a feeling there isn't a riflescope made that won't eventually fail. I equate it to a piece of granite. If you hit it with a hammer a few times nothing happens, but you hit it 50 or 60 times, something breaks."
In a typical shock test, the scope is clamped into a mechanism on a metal platform and connected to an accelerometer. A hammer is smashed into the table platform 50 or 100 times and then the scope is examined to see if the point of impact changes.
"Even in high-end products you will see failures. The lenses are held in with UV glue and that glue can crack," Highby said.
Another place a scope can fail is at the windage and elevation adjustments. Most scopes from top-name manufacturers have robust controls, but not all of them. Take a look at the dials with an eye to their durability.
A lot of today's riflescopes from competing manufacturers are made in the same factories. Optics are built to various specifications, depending on the label, but many of the processes are the same, and they are built with the same machines. Because Highby has been in a lot of the factories, he has an idea of the quality before he examines a scope.
"Usually we will know from past experiences who is making what product. If it is a well-known, good factory making stuff for some of the top names, there is a higher likelihood we would pick them up for the catalog."
If the scope is made in a factory that cuts corners, Highby is likely to be more skeptical. "We never want to disappoint our customers, so we take it serious. If it is a no-name vendor we are pretty cautious."
He gives it the old smell test, a secret trick of the trade. "It's a quick way to tell if it is high-end or mass-production scope. I take off the windage and elevation caps and sniff."
The scent of the factory lingers under the windage and elevation caps, Highby says. Fresh lubricant has "a clean acetone lab odor."
"A cheap scope smells like a bucket of used oil."